There is known a holographic method of monitoring the state of an elongated object (D. I. Mirovitski et al. "Mikrovolnovaya optika i golografiya", 1983, Nauka/Moscow/, pp.179, 202-206), comprising the steps of irradiating the surface of an object being monitored from a reference source of light, and subsequently registering, on a photosensitive material, the reference signal and the signal reflected by the surface of the monitored object, thus producing on the photosensitive material an interference pattern. The material and light source are placed in advance in the zone of monitoring. Following the fixation of the interference picture on the photosensitive material, a three-dimensional image of the monitored object is reproduced by irradiating the exposed material from the same light source.
There is also known an endoscopic method of monitoring the state of an elongated object (D. I. Mirovitski et al. "Mikrovolnovaya optika i golografiya", 1983, Nauka/Moscow/, p.215), comprising the main operations of the abovedescribed holographic method, plus an operation, following the fixation of the interference pattern on the photosensitive material, of transmitting this pattern via light guides for further processing the reproduction of the three-dimensional image of the monitored object.
The holographic and endoscopic monitoring methods can be performed by the known apparatus for monitoring the state of an elongated object (D. I. Mirovitski et al. "Mikrovolnovaya optika i golografiya", 1983, Nauka/Moscow/, pp. 205-206, 215), comprising a reference source of light and a photosensitive material so arranged in the zone of monitoring that the signal reflected by the surface of the object should strike the photosensitive material, and also light guides for conveying the interference pattern and a unit for processing the pattern for reproducing a three-dimensional image of the monitored object.
The holographic and endoscopic methods of monitoring the state of elongated objects, and the apparatus capable of performing these methods are distinguished by the following (D. I. Mirovitski et al. "Mikrovolnovaya optika i golografiya", 1983, Nauka/Moscow/, p.221). They are operable for monitoring objects of a length not exceeding several tens of centimeters, because when the length of an object exceeds 1 meter the reference light source and the photosensitive material should be movable along the object. This, however, substantially impairs the accuracy of fixation of the interference pattern, and, hence, the accuracy of measurement of the geometry of the reproduced three-dimensional image of the monitored object. Moreover, these methods and apparatus are suitable exclusively for monitoring the geometry of the surface of an objects, which drastically limits the capability of monitoring other variables and parameters.
The closest prior art of the disclosed solution by its technical essence is the method of monitoring the state of an elongated object, implemented for monitoring the geometry of marine risers in offshore drilling (AEG-Telefunken, BRD, "Position Measuring System for Offshore Installations. System design and mathematical description", 1980, 10 pp). The known method comprises the steps of selecting an element responsive to variation of the variable or parameter being monitored as representative of the state of the elongated object, selecting an extended line of transmission of wave energy carrying information on the variation of the monitored variable as representative of the state of the monitored object, matching the selected responsive element and extended line of transmission of wave energy, positioning the matched responsive element and extended line of transmission of wave energy in the zone of monitoring along a predetermined coordinate axis, along which the variable being controlled tends to vary, as representative of the state of the object being monitored, shaping and feeding to the input of the extended line of transmission of wave energy a time-modulated reference signal transformable as it propagates along the line in accordance with variation of the variable being monitored as representative of the state of the monitored object, measuring the parameters of the transformed reference signal at the output of the extended line of transmission of wave energy, and using the measured parameters of the transformed reference signal to determine the physical and mechanical characteristics of the state of the object being monitored along the predetermined coordinate axis of variation of the monitored variable. To be more specific, the operations of this method of prior art are implemented in the technique offered by AEG-Telefunken in the following manner.
For the monitored variables characterizing the state of the object being monitored--a marine riser, the technique of the prior art takes the angle of deviation of the axis of the riser from a vertical line, and twist angles in a horizontal plane at the top and bottom parts of the riser. Therefore, the selection of the responsive elements is carried out by providing specific sensors responsive to a variation of the said angles, i.e. of inertia inclinometers and magnetic compasses. This is followed by selecting the extended line of transmission of wave energy in the form of a shielded electric cable in a polyethylene protective sheating. The selected responsive elements and cable are matched by establishing induction coupling therebetween. The matched sensors and cable are positioned along the predetermined axis, i.e. along the generatrix of the marine riser, along which the angles being measured are presumed to vary. To obtain sufficient data on the geometry of the selected axis in a tree-dimensional space, the sensors of deviation of the axis from a vertical line are positioned and fixed along two orthogonal generatrices on the surface of the riser. Then a reference electric signal which is time-varying at a 400 Hz frequency is fed to the input of the cable. As the signal propagates along the cable, the induction coupling provides for supplying the sensors and obtaining from them the data on the angles of interest, but at a higher frequency than the carrier frequency of 400 Hz. The parameters of the transformed electric reference signals are measured at the output of the cable, the information is decoded, and the data on the angles of twist and deviation of the axis of the riser from the vertical line are interpreted as the physical and mechanical characteristics of the state of the riser from the distribution of the curvature and twist axially of the riser. The characteristics obtained determine unambiguously the strained state of the marine riser and the relative positions in space of its extreme points for dynamic positioning.
As far as the claimed apparatus of the presently disclosed technical solution is concerned, its closest prior art by the technical essence is the apparatus for monitoring the state of an elongated object, e.g. a marine riser in offshore drilling (AEG-Telefunken, BRD, "Position Measuring System for Offshore Installations. System design and mathematical description", 1980, 10 pp), comprising a source of modulated wave energy adapted to shape a time-varying reference signal, an extended line of transmission of wave energy, extending in the zone of monitoring the elongated object along a predetermined coordinate axis and being adapted for capturing and transmitting the data on variation of a variable being monitored, connected with the output of the source of modulated wave energy, a data processing unit adapted for extracting from the extended line of transmission of wave energy the reference signal transformed by this line and for determining the physical and mechanical characteristics of the state of th the elongated object, connected with the extended line of transmission of wave energy, and a video terminal adapted for presentation of the obtained physical and mechanical characteristics of the elongated object, having its input connected to the output of the data processing unit.
In the apparatus of the prior art, the source of modulated wave energy is in the form of a 400 Hz generator. The electric cable serves both for supplying the sensors positioned on the surface of the marine riser and for data transmission. For this purpose, the cable extends along the entire marine riser, its inner conductors being connected to the said generator and to a high-frequency (33-42 kHz) source of time-modulated oscillations required for interrogating the sensors. The cable has also connected to it the data processing unit in the form of a central station intended for extracting from the output of the cable the data supplied by the sensors and for converting them to a form susceptible to processing. The data processed by this station is sent for presentation onto the video terminal, i.e. onto a CRT display screen as the characteristics of the actual geometry of the axial line of the marine riser.
The following inherent disadvantages of the prior art method and apparatus for monitoring the state of an elongated object limit their applicability.
The said method and apparatus are unsuitable for monitoring elongated objects (e.g. structures) with high resolution and at considerable distances from the structure, on account of the limited information capacity of the line of transmission of wave energy, with the reliability of data transmission along the line being impaired with its length increasing, and the disturbance and voltage loss along this line growing accordingly. The employment of discretely positioned responsive elements in the form of angle sensors opposes in principle any continuous measurement of the distribution of a variable or parameter being monitored. It is possible only to approximate the distribution of the variable from the discretely obtained measurements along the predetermined coordinate axis, i.e. of the deviation angles axially of the marine riser from its strictly vertical position. Thus, the method and apparatus of the prior art are incapable in principle of providing for monitoring time- and -space continuous physical and mechanical characteristics of elongated objects, i.e. the curvature and axial twist of the riser, which results in approximation errors. Furthermore, the overall monitoring accuracy is also impaired by significant errors of the sensors themselves. The matching of a responsive member with the extended line of transmission of wave energy in the electromechanical manner through induction coupling and the specific fastening of the responsive elements to this line involves the positioning of sensors which are different in their operating principle, with a correspondingly complicated procedure of their interrogation, necessitating the provision and operation of costly underwater electronic equipment in an environment hazardous for its performance reliability.
Moreover, the employed sensors of the angle of deviation of the axis of the riser from verticality have a limited dynamic range of response to variation of the monitored angle (within 10.degree.), which prohibits their use for monitoring marine risers under conditions of considerable strain and deformation. Neither is it possible to employ the same method and apparatus for monitoring other kinds of elongated objects, or else to enhance the resolution of the measurements and monitoring without increasing the number of the sensors and reducing their spacing. It is worth mentioning, too, that the abovedescribed apparatus of the prior art for monitoring the state of an elongated objects consumes about 2 kW of electric power.